The known process for preparing polychlorofluoro-cyclopentenes (PCFCP) by reaction of halogen-containing cyclic compounds having a 5-membered ring with HF in the gas phase over catalysts use, as the starting material, either octachlorocyclopentene (DE-OS) (German Published Specification) 1,543,015) or partly fluorinated chloro-fluoro-cyclopent-1-enes which are chlorinated on the double bond and have at least one fluorine atom in the molecule (U.S. Pat. No. 3,178,482; and U.S. Pat. No. 3,258,500). Catalysts which are employed for such reactions are active charcoals (U.S. Pat. No. 3,178,482), activated chromium gels (U.S. Pat. No. 3,258,500) or aluminum oxide doped with copper or cobalt (DE-OS (German Published Specification) 1,543,015). The processes mentioned allow only moderate conversions to PCFCP and start from starting substances which themselves are accessible only via multi-stage synthesis.
According to U.S. Pat. No. 2,449,233, hexachlorocyclopentadiene is converted into PCFCP in the liquid phase under pressure with SbF.sub.5 produced in situ from SbCl.sub.5 and HF/Cl.sub.2. This process is laborious and requires large amounts of expensive Sb compounds which pollute the environment.
DE-OS (German Published Specification) 1,900,241 describes the hydrofluorination of partly and completely chlorinated olefines in the gas phase over Sb-containing supported catalysts on Al.sub.2 O.sub.3, which is said to lead to the corresponding saturated fluoroalkanes. In this DE-OS (German Published Specification), hexachlorocyclopentadiene is mentioned as the cyclic halogenohydrocarbon on page 6, line 8, but a description of the reaction conditions and the products formed is absent; thus, above all no PCFCP is mentioned as a product.
It is furthermore known from DE-OS (German Published Specification) 2,057,398 that aliphatic chlorohydrocarbons can be split in the absence of catalysts with Cl.sub.2 and HF at temperatures of 400.degree. to 800.degree. C. to give carbon tetrachloride or fluorochloromethanes. According to page 3, line 22 of this DE-OS (German Published Specification), this also applies to perchlorinated cycloaliphatics.
It was thus surprising that the present process according to the invention allows chlorine-fluorine exchange on hexachlorocyclopentadiene to form halogenated cyclopentenes of varying degrees of fluorination. This is because DE-OS (German Published Specification) 1,543,015 states on page 12, line 13, that more highly (completely) halogenated (cyclo)-aliphatics can be fluorinated more rapidly, so that this DE-OS (German Published Specification) considers the octachloro-cyclopentene which is more troublesome to prepare to be necessary, although this also produces only moderate results. The starting substance hexachlorocyclopentadiene is an industrially accessible compound which can be prepared conventionally (Ullmanns Enzyklopadie der technischen Chemie (Ullmann's Encyclopaedia of Industrial Chemistry), 5th edition, Volume A8 (1987), 231; and Chem. Rev. 58 (1958),250).